Duplex stainless steel

Duplex stainless steels are called “duplex” because they have a two-phase microstructure consisting of grains of ferritic and austenitic stainless steel. The picture shows the yellow austenitic phase as “islands” surrounded by the blue ferritic phase. When duplex stainless steel is melted it solidifies from the liquid phase to a completely ferritic structure. As the material cools to room temperature, about half of the ferritic grains transform to austenitic grains (“islands”). The result is a microstructure of roughly 50% austenite and 50% ferrite.

Duplex stainless steels have a two-phase microstructure of austenite and ferrite grains.

The duplex structure gives this family of stainless steels a combination of attractive properties:

Strength: Duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels.

Toughness and ductility: Duplex stainless steels have significantly better toughness and ductility than ferritic grades; however, they do not reach the excellent values of austenitic grades.

Stress corrosion cracking resistance: Duplex stainless steels show very good stress corrosion cracking (SCC) resistance, a property they have “inherited” from the ferritic side. SCC can be a problem under certain circumstances (chlorides, humidity, elevated temperature) for standard austenitics such as Types 304 and 316.

Cost: Duplex stainless steels have lower nickel and molybdenum contents than their austenitic counterparts of similar corrosion resistance. Due to the lower alloying content, duplex stainless steels can be lower in cost, especially in times of high alloy surcharges. Additionally, it may often be possible to reduce the section thickness of duplex stainless steel, due to its increased yield strength compared to austenitic stainless steel. The combination can lead to significant cost and weight savings compared to a solution in austenitic stainless steels.

Please download the updated brochure "Practical Guidelines for the Fabrication of Duplex Stainless Steels" for more information.

Wireline is cable used to lower oil- and gas-well tools and measuring equipment downhole. Wireline must be strong, dependable and resistant to the increasingly corrosive conditions encountered in today’s deeper wells. Molybdenum imparts the required corrosion resistance to the stainless steel and nickel alloys used in this application.

Decorative stainless steel panels are widely used in façades, roofs and elevators because they are attractive and long-lasting. Increasingly, engineers now use stainless steel for load-bearing structures in challenging environments. In many cases stainless steel is cost-effective if the whole life cycle of the installation is considered.

A recent study found that about one quarter of the molybdenum used each year is recycled material from scrap sources. The rest is newly mined, primary molybdenum. Scrap therefore plays an important role in meeting demand and contributing to sustainability.

Material longevity and its inherent impact on long-term project sustainability are changing the way buildings are designed. Memorials like the new Four Freedoms Park in New York have to last for hundreds of years. Corrosion resistant 2205 duplex stainless steel was necessary so that the sculptural handrails were as durable as the massive blocks of granite in this highly-acclaimed new monument.

A major historic ship preservation project is currently under way in Stockholm. The galvanized and epoxy coated mild steel bolts used to hold the ship together after it was raised are failing due to corrosion. Molybdenum is a key alloying element in the high-strength, corrosion-resistant super duplex stainless steel Sandvik SAF 2507TM and Sandvik SAF 2707 HDTM bolts that will hold the great ship Vasa together to achieve a minimum design life of 100 years.